1. Field of the Invention
The present invention relates to a semiconductor device including a resistor element doped with impurity.
2. Description of Related Art
A thermal type flow sensor for detecting a flow amount of fluid has been known hitherto. This type of flow sensor includes a heater element and a temperature-sensitive element, both made by doping impurity into silicon semi-conductor layer. Examples of this type of sensor are shown in Japanese Patents No. 3468731 and No. 3698679. In this sensor device, a heater element and a temperature-sensitive element are made by doping impurity into a semiconductor layer. Both of the heater element and the temperature-sensitive element are resistor elements, and the specific resistance thereof is made 8×10−4 Ωcm or lower, and an impurity density is made 2×1020/cm3 or higher. In another example shown in JP-A-2004-205498, the impurity density in the resistor element made by doping the impurity into a semiconductor layer is set to 1×1020/cm3 or higher.
Tests conducted by inventors of the present invention revealed the fact that a resistance change by aging in the resistor having a high impurity density is high when the resistor is at a high temperature, e.g., a temperature higher than 310° C.